1. Field of the Invention
The present invention relates generally to a method and apparatus for continuously annealing a metal strip. More specifically, the invention relates to an improved hearth roll arrangement for an annealing furnace. The present invention also relates to a hearth roll for the improved annealing furnace.
2. Description of the Background Art
As is well known, a plurality of hearth rolls are arranged in a heat treatment furnace such as annealing furnace at vertically offset positions for defining a zig-zag path for a metal strip. The metal strip is fed through the path at a given speed and heated at a predetermined heat cycle. Usually, the metal strip is fed through the metal strip path thus defined, at such a high speed that the LSD value (line speed (mpm).times.strip thickness (mm)) is greater than or equal to 150 (mpm.mm). Such high speed feed of the metal strip tends to cause slippage between the hearth roll and the metal strip resulting in meandering of the strip. Meandering of the metal strip is apt to cause breakage, cracking or so forth of the strip.
In order to prevent slipping between the hearth roll and the metal strip, the peripheral surface of the hearth roll, which contacts the surface of the metal strip, is treated to have a predetermined roughness. In order to maintain sufficient grip between the hearth roll peripheral surface and the metal strip surface and thereby prevent slippage therebetween, the peripheral surface of the hearth roll should be provided satisfactory wear resistance. Therefore, wear-resistant surface treatment has to be performed on the peripheral surface of the hearth roll. Conventionally, this surface treatment has been performed by spray coating of ceramics, such as chromium carbide, tungsten carbide. Hearth rolls having a ceramic layer formed thereon by spray coating will be hereafter referred to as "ceramic coated roll" throughout the disclosure.
Such ceramic coated roll exhibits a satisfactorily high wear resistance and heat resistance for use in an annealing furnace. On the other hand, such ceramic coated rolls have some drawbacks. For example, it is difficult to maintain a consistant surface roughness on the ceramic coated roll and fluctuation of the roughness on the peripheral surface of the hearth roll tends to occur. Another drawback of the ceramic coated roll is the relatively high possibility of adherence of welding slag or oxidation scale on the coated surface. Such adherence tends to form impressions on the metal strip to be treated. Furthermore, such ceramic coated rolls are rather expensive in comparison with plated rolls, such as a chrome plated roll. Rolls whose peripheral surface is plated, such as with chrome plating, will be hereafter referred to as "plated roll" throughout the disclosure.
Such plated rolls, especially the chrome plated rolls have been considered as not applicable for the annealing furnace due to softening at high temperature. That is, in the case of the chrome plated roll, the hardness of the chrome plating layer drops significantly when the temperature increases above 450.degree. C. On the other hand, at temperatures lower than 450.degree. C., such a chrome plating layer exhibits hardness equivalent to that of the ceramic layer on ceramic coated rolls. In addition, it is easier to control the surface roughness of plated rolls than ceramic coated rolls.